1. Field of the Invention
The present invention relates to a method for preventing reduction in a reproduced output of a servo signal in a magnetic recording medium, especially in a perpendicular magnetic recording medium in a discrete track disk type.
2. Description of the Related Art
In a magnetic recording medium such as a hard disk, a higher areal density has been achieved by virtue of the increase in linear recording density and track recording density. These recording densities are essential factors to further increase the areal density in the future.
A perpendicular magnetic recording technique has been used to further increase the linear recording density. A perpendicular magnetic recording medium used in this technique can achieve a higher linear recording density due to its ability, for example, to ensure stability against thermal fluctuation of magnetization of a recording medium, as compared with a conventional longitudinal magnetic recording medium which has been widely used.
In this perpendicular magnetic recording technique which can increase the linear recording density as described above, a higher track recording density can be further realized by forming predetermined concave/convex patterns on a perpendicular magnetic recording medium, and by recording data signals and servo signals on the convex patterns. The perpendicular magnetic recording medium used in this technique is called a “discrete track disk type.” See, for example, the specifications etc. of Japanese Patent Laid-open Publication No. 328662/99 and 195042/2000, for details.
In the perpendicular magnetic recording medium in this discrete track disk type, since the adjacent magnetic recording tracks on the convex patterns on which data signals are recorded are separated by concave patterns, the data signals are less affected by the adjacent magnetic recording tracks in both read and write processes. This allows an increase in track recording density of the perpendicular magnetic recording medium in the discrete track disk type, compared to a perpendicular magnetic recording medium in a continuous medium disk type.
However, it is known in the art that the increase in the areal density of a magnetic recording medium causes the phenomenon of so-called thermal fluctuation to occur in a greater degree. This phenomenon, which leads to a gradual reduction in magnetization, is caused by stochastic fluctuation in magnetization due to disturbance of thermal energy, and makes it difficult to increase the areal density.
The thermal fluctuation will be described next. As the areal density of a magnetic recording medium is increased, the size of magnetic recording bits becomes smaller in the magnetic recording medium. In order to ensure a high Signal/Noise ratio even with smaller magnetic recording bits, the magnetic recording layer must consist of ferromagnetic grains in more than a specific number. In order to ensure a specific number of ferromagnetic grains, the ferromagnetic grains must be reduced in size. However, if the ferromagnetic grains are reduced in size, the disturbance of thermal energy acts on the recording medium as a disturbance, thereby causing the stochastic fluctuation of the magnetization. Thus, the phenomenon of thermal fluctuation occurs to an even greater degree.
For example, in a longitudinal magnetic recording medium conventionally used in various applications, thermal fluctuation arises in an even greater degree, when the ratio KuV/kbT is less than about 60. For reference, KuV/kbT is the ratio of magnetization energy of ferromagnetic grains KuV, to thermal energy at an ambient temperature kbT, where Ku is the magnetic anisotropy constant, V is the volume of the ferromagnetic grains, kb is the Boltzmann's constant, and T is absolute temperature.
In a longitudinal magnetic recording medium, the magnetization is reduced in a greater degree in a higher recording density region due to thermal fluctuation, because the demagnetizing field, which reduces the magnetization, increases in the higher recording density region. Therefore, a data region in which data signals are recorded tends to be affected to a greater degree by the thermal fluctuation than a servo region in which servo signals are recorded, because the data region has a higher recording density and the servo region has a lower recording density. A method for preventing reduction in the magnetization in a data region is proposed in the specification etc. of Japanese Patent Laid-open Publication No. 110004/2001.
On the other hand, in a perpendicular magnetic recording medium, contrary to the longitudinal recording medium, as the recording density is increased, the magnetization becomes more stable, and the medium is less affected by the thermal fluctuation. In other word, as the recording density is lowered, or the bit length is increased, the demagnetizing field which reduces the magnetization is increased. As a result, the medium tends to be affected in a greater degree by the thermal fluctuation, and the magnetization is degraded. Therefore, in a perpendicular magnetic recording medium, it is the servo region, which has a relatively low recording density, that is affected most by the thermal fluctuation.
However, the servo signal is not refreshed again after being recorded once. Since a perpendicular magnetic recording medium is affected in the servo region in a greater degree due to thermal fluctuation, accumulation of the thermal fluctuation in the servo region over a long time may result in worse influence on the tracking performance of a magnetic head.
The thermal fluctuation in the servo region of the perpendicular magnetic recording medium is still more critical in the above-described discrete track disk type, compared to the continuous medium disk type, for the following reason.
Referring to FIG. 1, a servo region in a perpendicular magnetic recording medium in the discrete track disk type is illustrated in a schematic diagram. In a servo region in the perpendicular magnetic recording medium in the discrete track disk type, servo signals are recorded by uniformly magnetizing the medium in one direction perpendicular to the medium surface. As a result, convex patterns in which servo signals are recorded, are uniformly magnetized in either the upward or the downward direction. In FIG. 1, all of the convex patterns are magnetized in the upward direction. The convex patterns in which servo signals are recorded, are separated bit by bit with the concave patterns in the circumferential direction of the disk.
Referring next to FIG. 2, a servo region in a perpendicular magnetic recording medium in the continuous medium disk type is illustrated in a schematic diagram. Unlike the perpendicular magnetic recording medium in the discrete track disk type which is entirely magnetized in one direction, servo signals are recorded bit by bit such that the adjacent bits are magnetized in anti-parallel directions to each other. Further, unlike the perpendicular magnetic recording medium in the discrete track disk type, the bits are not spatially separated from each other in the circumferential direction of the disk in a sector.
Since the perpendicular magnetic recording medium in the discrete track disk type is not magnetized in anti-parallel directions as the perpendicular magnetic recording medium in the continuous medium disk type, the amplitude of a reproduced servo signal is approximately half of the continuous medium disk type. This means that the perpendicular magnetic recording medium in the discrete track disk type, the output amplitude of which is inherently low, is even more affected when the output amplitude is lowered by thermal fluctuation.
Further, unlike the perpendicular magnetic recording medium in the continuous medium disk type, the perpendicular magnetic recording medium in the discrete track disk type tends to be more affected by the demagnetizing field due to the isolated bits, resulting in unstable magnetization. In other words, the perpendicular magnetic recording medium in the discrete track disk type is more susceptible to the influence of thermal fluctuation than the perpendicular magnetic recording medium in the continuous medium disk type.
Thus, there exists a strong need for reducing the influence of thermal fluctuation in the perpendicular magnetic recording medium in the discrete track disk type, than the perpendicular magnetic recording medium in the continuous medium disk type.